Multi-pulse constant voltage transformer for a variable speed drive in chiller applications

ABSTRACT

A multi-pulse transformer with multiple taps provides a constant magnitude voltage output to a variable speed chiller&#39;s compressor motor over a range of input voltages. The 3-phase transformer includes primary windings and a plurality of secondary windings. The secondary windings are electromagnetically coupled with the associated primary winding. The primary windings include taps for receiving multiple input AC voltages and the secondary windings have a single output terminal for supplying a predetermined output voltage which, after rectification produces a DC multi-pulse waveform for powering a DC link of a variable speed drive. Alternatively the 3-phase transformer includes multiple taps on the secondary windings. Each of the primary windings has a terminal for receiving an input AC voltage. The taps of the secondary windings provide an output voltage that is converted to a multi-pulse waveform for powering a DC link of a variable speed drive.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims priority to and the benefit of U.S.Provisional Patent Application No. 61/934,918 filed Feb. 3, 2014,entitled “MULTI-PULSE CONSTANT VOLTAGE TRANSFORMER FOR A VARIABLE SPEEDDRIVE IN CHILLER APPLICATIONS”, which is hereby incorporated byreference.

BACKGROUND

The application generally relates to a multiple pulse, or multi-pulse,transformer. The application relates more specifically to a multi-pulsetransformer with multiple taps to provide a constant magnitude voltageoutput to a variable speed chiller's compressor motor over a range ofinput voltages.

AC-to-DC converters play a significant role in the modern energyconversion industry. Multi-pulse transformers (18 pulse, 24 pulse orothers) along with full wave rectifiers have been used to reduce inputcurrent harmonic and meet industry guidelines for limits on voltagedistortion, e.g., as may be caused by harmonics generated in theconverter.

Manufacturers of computers and other digital equipment such asprogrammable controllers may require AC power sources having a harmonicdistortion factor of no more than a 5%, and the largest single harmonichaving no more than 3% of the rated RMS line voltage. Higher levels ofharmonics may result in malfunctions of peripheral equipment that canhave serious consequences. Instruments can be affected by providingerroneous data or otherwise performing outside the design criterion.

Intended advantages of the disclosed systems and/or methods satisfy oneor more of these needs or provide other advantageous features. Otherfeatures and advantages will be made apparent from the presentspecification. The teachings disclosed extend to those embodiments thatfall within the scope of the claims, regardless of whether theyaccomplish one or more of the aforementioned needs.

SUMMARY

One embodiment relates to a variable speed drive system that receives aninput AC power at a fixed AC input voltage magnitude and frequency andprovides an output AC power at a variable voltage and variablefrequency. The variable speed drive includes a multiple pulsetransformer connected to a three-phase AC power source providing theinput AC voltage. A converter converts the input AC voltage to a DCvoltage. A DC link is connected to the converter stage. The DC linkfilters and stores the DC voltage from the converter stage. An inverteris connected to the DC link to convert the DC voltage from the DC linkinto the output AC power. The multiple pulse transformer includes threeprimary windings. Each of the primary windings is connected to a phaseof the three-phase AC power source. Each of the primary windingsincludes a plurality of input taps for connection to the AC powersource. The multiple pulse transformer also includes a plurality ofsecondary windings. Each secondary winding includes three phase windingsrespectively. Each phase winding of the respective secondary winding hasa predetermined phase shift with respect to a corresponding phasewinding of the remaining secondary windings. The phase shifting of thephase windings results in three sinusoidal output voltage waves for eachsecondary winding. The sinusoidal output voltage waves are substantiallyevenly spaced over 360 degrees.

Another embodiment relates to a variable speed drive system thatreceives an input AC power at a fixed AC input voltage magnitude andfrequency and provide an output AC power at a variable voltage andvariable frequency. The variable speed drive includes a multiple pulsetransformer connected to a three-phase AC power source providing theinput AC voltage. A converter configured to convert the input AC voltageto a DC voltage. A DC link is connected to the converter. The DC linkfilters and stores the DC voltage from the converter. An inverter isconnected to the DC link to convert the DC voltage from the DC link intothe output AC power having variable voltage and variable frequency. Themultiple pulse transformer includes three primary windings connected tothe three-phase AC power source. Each primary winding includes at leastone input tap for connection to the AC power source. The multiple pulsetransformer also includes a plurality of secondary phase windings. Eachsecondary phase winding includes multiple voltage output terminals.

Still another embodiment relates to a chiller system including arefrigerant circuit. The refrigerant circuit includes compressor, acondenser, and an evaporator connected in a closed refrigerant loop. Amultiple pulse transformer includes three primary windings connected toa three-phase AC power source and a plurality of secondary windingsconnected to a variable speed drive to power a motor of the compressor.The primary windings or secondary windings include multiple taps forproviding a multiple phase output voltage with a predetermined phaseshift with respect to a corresponding winding of the remaining primaryor secondary windings. The phase shifting of the multiple phase outputvoltage provides three multiple sinusoidal output voltage waves for eachof the plurality of secondary windings, with the multiple sinusoidaloutput voltage waves being substantially evenly spaced over 360 degrees.

An advantage of the embodiments described herein include a multi-pulsetransformer that has multiple taps either on the input winding or on theoutput winding such that AC output voltage with constant magnitude isobtained under different input voltage levels, for example 380V inputvoltage, 460V input voltage or etc.

Another advantage is a constant value of AC output voltage may beprovided from a DC link in a variable speed drive with an optimal DClink voltage value. The DC link voltage is obtained after the AC inputvoltage is applied to multiple full wave rectifiers.

Another advantage is that for low voltage (600 Volts AC or less) HVACapplications, a 575V 60 Hz motor may be used for all 380 to 600 Volt AC50 and 60 Hz applications, which allows one family of motors for allapplications globally, is most cost effective; maximizes the outputrating in horsepower for a given insulated gate bipolar transistor(IGBT) inverter power pole; and minimizes the cost of the motor. This isbecause IGBTs are rated in amps, so a higher voltage motor increases theoutput horsepower rating. The use of a 575 V, four pole motor that isdriven over a range of at least twice the power line input frequencycould further reduce system cost and size for a given horsepower rating.For all input voltages ranging from 380 to 600 Volts AC, 50 and 60 Hz,if all compressor motors selected are rated at 575 Volts, 60 Hz, the DClink voltage could be fixed at 820V.

A further advantage of the disclosed multi-pulse constant voltagetransformer is to simplify variable speed drive design in globalapplications, since utility grid voltages vary from country to country.

Still other advantages provide by the use of a multi-pulse transformerin a variable speed drive include reduced input current harmonics,improved immunity to power quality issues, and increased efficiency overPWM rectifiers. Transformers are generally more efficient than PWMrectifiers since a PWM rectifier has switching and conducting lossesassociated with the IGBTs or other power semiconductor switchingdevices.

Alternative exemplary embodiments relate to other features andcombinations of features as may be generally recited in the claims.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an exemplary embodiment for a heating, ventilation and airconditioning system.

FIG. 2 shows an isometric view of an exemplary vapor compression system.

FIG. 3 shows schematically an exemplary embodiment for a heating,ventilation and air conditioning system.

FIG. 4 shows schematically an exemplary embodiment of a variable speeddrive.

FIG. 5 shows an exemplary multi-pulse transformer with multiple tapprimary windings.

FIG. 6 shows an exemplary multi-pulse transformer with multiple tapsecondary windings.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

FIG. 1 shows an exemplary environment for a heating, ventilation and airconditioning (HVAC) system 10 in a building 12 for a typical commercialsetting. System 10 can include a vapor compression system 14 that cansupply a chilled liquid which may be used to cool building 12. System 10can include a boiler 16 to supply a heated liquid that may be used toheat building 12, and an air distribution system which circulates airthrough building 12. The air distribution system can also include an airreturn duct 18, an air supply duct 20 and an air handler 22. Air handler22 can include a heat exchanger that is connected to boiler 16 and vaporcompression system 14 by conduits 24. The heat exchanger in air handler22 may receive either heated liquid from boiler 16 or chilled liquidfrom vapor compression system 14, depending on the mode of operation ofsystem 10. System 10 is shown with a separate air handler on each floorof building 12, but it is appreciated that the components may be sharedbetween or among floors.

FIGS. 2 and 3 show an exemplary vapor compression system 14 that can beused in HVAC system 10. Vapor compression system 14 can circulate arefrigerant through a circuit starting with compressor 32 and includinga condenser 34, expansion valve(s) or device(s) 36, and an evaporator orliquid chiller 38. Vapor compression system 14 can also include acontrol panel 40 that can include an analog to digital (A/D) converter42, a microprocessor 44, a non-volatile memory 46, and an interfaceboard 48. Some examples of fluids that may be used as refrigerants invapor compression system 14 are hydrofluorocarbon (HFC) basedrefrigerants, for example, R-410A, R-407, R-134a, hydrofluoro olefin(HFO), “natural” refrigerants like ammonia (NH₃), R-717, carbon dioxide(CO₂), R-744, or hydrocarbon based refrigerants, water vapor or anyother suitable type of refrigerant.

Motor 50 used with compressor 32 can be powered by a variable speeddrive (VSD) 52 or can be powered directly from an alternating current(AC) or direct current (DC) power source. Motor 50 can include aninduction, or synchronous motor, or switched reluctance (SR) motor, orpermanent magnet synchronous motor (PMSM) that can be powered by a VSDor directly from an AC or DC power source.

FIG. 4 shows an exemplary embodiment of a VSD including a multiple pulsetransformer 100. VSD 52 receives AC power having a particular fixed linevoltage and fixed line frequency from an AC power source and provides ACpower to motor 50 at a desired voltage and desired frequency, both ofwhich can be varied to satisfy particular requirements. VSD 52 can havefour components: a multiple pulse transformer, a rectifier/converter222, a DC link 224 and an inverter 226. The multiple pulse transformertransforms a 3-phase input AC voltage into multiple AC or sinusoidalvoltage waveforms as described in greater detail below. Therectifier/converter 222 converts the multiple sinusoidal voltagewaveforms from the multiple pulse transformer 100 into DC voltage. TheDC link 224 filters the DC power from the converter 222 and providesenergy storage components such as capacitors and/or inductors. Finally,inverter 226 converts the DC voltage from DC link 224 into variablefrequency, variable magnitude AC voltage for motor 50.

In an exemplary embodiment, the rectifier/converter 222 may be athree-phase pulse width modulated boost rectifier having insulated gatebipolar transistors to provide a boosted DC voltage to the DC link 224to obtain a maximum RMS output voltage from VSD 52 greater than theinput voltage to VSD 52. Alternately, the converter 222 may be a passivediode or thyristor rectifier without voltage-boosting capability.

VSD 52 can provide a variable magnitude output voltage and variablefrequency to motor 50, to permit effective operation of motor 50 inresponse to a particular load conditions. Control panel 40 can providecontrol signals to VSD 52 to operate the VSD 52 and motor 50 atappropriate operational settings for the particular sensor readingsreceived by control panel 40. For example, control panel 40 can providecontrol signals to VSD 52 to adjust the output voltage and outputfrequency provided by VSD 52 in response to changing conditions in vaporcompression system 14, i.e., control panel 40 can provide instructionsto increase or decrease the output voltage and output frequency providedby VSD 52 in response to increasing or decreasing load conditions oncompressor 32. The estimated rotor phase angle θ_(r) and rotor frequencyω_(r), of motor 50, as described in more detail below, may be input tothe control panel for feedback control of the position and rotationalfrequency of motor 50.

Compressor 32 compresses a refrigerant vapor and delivers the vapor tocondenser 34 through a discharge passage. In one exemplary embodiment,compressor 32 can be a centrifugal compressor having one or morecompression stages. The refrigerant vapor delivered by compressor 32 tocondenser 34 transfers heat to a fluid, for example, water or air. Therefrigerant vapor condenses to a refrigerant liquid in condenser 34 as aresult of the heat transfer with the fluid. The liquid refrigerant fromcondenser 34 flows through expansion device 36 to evaporator 38. A hotgas bypass valve (HGBV) 134 may be connected in a separate lineextending from compressor discharge to compressor suction. In theexemplary embodiment shown in FIG. 3, condenser 34 is water cooled andincludes a tube bundle 54 connected to a cooling tower 56.

The liquid refrigerant delivered to evaporator 38 absorbs heat fromanother fluid, which may or may not be the same type of fluid used forcondenser 34, and undergoes a phase change to a refrigerant vapor. Inthe exemplary embodiment shown in FIG. 3, evaporator 38 includes a tubebundle 60 having a supply line 60S and a return line 60R connected to acooling load 62. A process fluid, for example, water, ethylene glycol,calcium chloride brine, sodium chloride brine, or any other suitableliquid, enters evaporator 38 via return line 60R and exits evaporator 38via supply line 60S. Evaporator 38 lowers the temperature of the processfluid in the tubes. The tube bundle 60 in evaporator 38 can include aplurality of tubes and a plurality of tube bundles. The vaporrefrigerant exits evaporator 38 and returns to compressor 32 by asuction line to complete the circuit or cycle. In an exemplaryembodiment, vapor compression system 14 may use one or more of each ofvariable speed drive (VSD) 52, motor 50, compressor 32, condenser 34,expansion valve 36 and/or evaporator 38 in one or more refrigerantcircuits.

Referring next to FIG. 5, an exemplary multi-pulse transformer 100includes input or primary windings 102 a, 102 b and 102 c for connectionto a three-phase input power source 104. Each of input windings 102 a,102 b and 102 c includes multiple terminals for connection to inputpower source 104. In the exemplary embodiment shown in FIG. 5 there arethree terminals on each input winding, indicated as terminals A, A1 andA2 on winding 102 a; terminals B, B1 and B2 on winding 102 b, andterminals C, C1 and C2 on winding 102 c.

Transformer 100 further includes output or secondary windings 104, 106and 108, each of which has three phase windings designated as a, b andc, respectively. As indicated in FIG. 5, secondary phase winding 104 ahas a phase shift of θ with respect to secondary phase winding 106 a,and secondary winding 108 a has a phase shift of −θwith respect tosecondary winding 106 a. Similarly each of the respective outputwindings 104 b, 106 b, and 108 b is phase shifted +/−θ with respect tothe adjacent phase, and each of output windings 104 c, 106 c and 108 cis phase shifted +/−θ. Phase shifting of secondary windings 104, 106 and108 and their respective phase windings results in nine sine waves atthe output terminals D, E, F, respectively of phase windings 104 a, b &c; 106 a, b & c, and 108 a, b & c, respectively at substantially equallyspaced phase angles, or at about 40° angles of separation betweenvoltage peaks. When rectified by rectifier/converter 222, the resultingwave form results in an 18 pulse DC waveform supplied to DC link 224.While the exemplary embodiments as described are for 18 pulse (9 phase)output waveform, it should be understood that other configurations,e.g., 12-pulse (6-phase), 24-pulse (12-phase), as desired to achieve thedesired ripple factor available at the DC link.

In the exemplary embodiment shown in FIG. 5, input windings 102 may bewound for input terminals arranged as follows:

575 Volts→terminals A, B & C

460 Volts→terminals A1, B1 & C1

400/415 Volts→terminals A2, B2 & C2

Thus, depending on the utility voltage available at input power source104, any of the three voltages above may be applied at the respectiveterminals to provide the same output voltage at the output terminals ofsecondary phase windings, e.g., 580 VAC RMS voltage in order to convertthe output voltage of secondary phase windings to 820 VDC on the DClink. As indicated above, it is common to use a 575 volt motor for HVACapplications to maximize output power for a given IGBT inverter powerpole. A 575 V, four pole motor may be driven over a range of at leasttwice the power line input frequency to reduce system cost and size fora given motor horsepower rating. For a 575V induction machine, thepreferred DC link voltage is 820V, although transformer 100 may bedesigned to provide various DC bus voltages, e.g., in a range of 813V upto 1000V, suitable for motor voltage ratings of 575 volts.

Referring next to FIG. 6, in an alternate embodiment an exemplarymulti-pulse transformer 200 having secondary windings with multiple tapsis shown. In another embodiment multiple taps may be provided on theoutput winding 204, 206, 208 such that AC output voltage is maintainedwith constant magnitude is obtained under different input voltagelevels, for example 380V input voltage, 460V input voltage or etc. In anembodiment where secondary windings include multiple taps, primarywindings 202 may include only one terminal connection to the inputvoltage. Alternately primary windings 202 may also include multipletaps.

Multi-pulse transformer 200 includes input or primary windings 202 with3-primary phase windings 202 a, 202 b and 202 c for connection to athree-phase input power source 202. As shown in FIG. 6, each inputwinding 202 a, 202 b and 202 c includes a single input terminal A_(i),B_(i), C_(i), respectively, for connection to input power source 202.

Transformer 200 further includes output or secondary windings 204, 206and 208, each of which has three output phase windings with multipleterminals. On output winding 204, phase A terminals are designated asA_(O1), A_(O11), and A_(O12); phase B terminals as B_(O1), B_(O11), andB_(O12) and phase C terminals as C_(O1), C_(O11), and C_(O12),respectively. On output winding 206, phase A terminals are designated asA_(O2), A_(O21), and A_(O22;) phase B terminals as B_(O2), B_(O21), andB_(O22) and phase C terminals as C_(O2), C_(O21), and C_(O22),respectively; On output winding 208, phase A terminals are designated asA_(O3), A_(O31), and A_(O32); phase B terminals as B_(O3), B_(O31), andB_(O32) and phase C terminals as C_(O3), C_(O31), and C_(O32),respectively. As indicated in FIG. 6, secondary phase winding 204A has aphase shift of θ with respect to secondary phase winding 206A, andsecondary winding 208A has a phase shift of −θ with respect to secondarywinding 206A. Similarly each of the respective output windings 204B,206B, and 208B is phase shifted +/−θ with respect to the adjacent phase,and each of output windings 204C, 206C and 208C is phase shifted +/−θwith respect to the adjacent phase. Phase shifting of secondary windings204, 206 and 208 and their respective phase windings results in ninesine waves at the output terminals D, E, F, respectively of phasewindings 204 A, B & C; 206 A, B & C, AND 208 A, B & C, respectively atsubstantially equally spaced phase angles, or at about 40° angles ofseparation between voltage peaks. When rectified by rectifier/converter222, the resulting wave form results in an 18 pulse waveform supplied toDC link 224.

In the exemplary embodiment shown in FIG. 6, input windings 202 may bewound for a standard input AC voltage, and the output terminals arrangedto output 575 Volts at one of the 3 terminal options, depending on thestandard input AC voltage, e.g., 575V, 460V or 400/415V, applied atterminals 202 A_(i), B_(i), C_(i).

Thus, depending on the utility voltage provided at input power source202, any of the three voltages above applied at the input terminals willprovide the desired output voltage at one the output terminalcombinations of secondary phase windings, e.g., 575 volts for a 575Vmotor. As indicated above, it is common to use a 575 volt motor for HVACapplications to maximize output power for a given IGBT inverter powerpole.

It should be understood that the application is not limited to thedetails or methodology set forth in the following description orillustrated in the figures. It should also be understood that thephraseology and terminology employed herein is for the purpose ofdescription only and should not be regarded as limiting.

While the exemplary embodiments illustrated in the figures and describedherein are presently preferred, it should be understood that theseembodiments are offered by way of example only. Accordingly, the presentapplication is not limited to a particular embodiment, but extends tovarious modifications that nevertheless fall within the scope of theappended claims. The order or sequence of any processes or method stepsmay be varied or re-sequenced according to alternative embodiments.

It is important to note that the construction and arrangement of themulti-pulse transformer as shown in the various exemplary embodiments isillustrative only. Although only a few embodiments have been describedin detail in this disclosure, those who review this disclosure willreadily appreciate that many modifications are possible (e.g.,variations in sizes, dimensions, structures, shapes and proportions ofthe various elements, values of parameters, mounting arrangements, useof materials, colors, orientations, etc.) without materially departingfrom the novel teachings and advantages of the subject matter recited inthe claims. For example, elements shown as integrally formed may beconstructed of multiple parts or elements, the position of elements maybe reversed or otherwise varied, and the nature or number of discreteelements or positions may be altered or varied. Accordingly, all suchmodifications are intended to be included within the scope of thepresent application. The order or sequence of any process or methodsteps may be varied or re-sequenced according to alternativeembodiments. In the claims, any means-plus-function clause is intendedto cover the structures described herein as performing the recitedfunction and not only structural equivalents but also equivalentstructures. Other substitutions, modifications, changes and omissionsmay be made in the design, operating conditions and arrangement of theexemplary embodiments without departing from the scope of the presentapplication.

1. A variable speed drive system configured to receive an input AC powerat a fixed AC input voltage magnitude and frequency and provide anoutput AC power at a variable voltage and variable frequency, thevariable speed drive comprising: a multiple pulse transformer, connectedto a three-phase AC power source providing the input AC voltage; aconverter configured to convert the input AC voltage to a DC voltage; aDC link connected to the converter stage, the DC link being configuredto filter and store the DC voltage from the converter stage; and aninverter connected to the DC link, the inverter being configured toconvert the DC voltage from the DC link into the output AC power havingthe variable voltage and the variable frequency; the multiple pulsetransformer comprising: three primary windings, each of the primarywindings connected to a phase of the three-phase AC power source; eachof the primary windings comprising a plurality of input taps forconnection to the AC power source; a plurality of secondary windings,each secondary winding comprising three phase windings respectively;each phase winding of the respective secondary winding comprising apredetermined phase shift with respect to a corresponding phase windingof the remaining secondary windings; wherein the phase shifting of thephase windings results in three sinusoidal output voltage waves for eachsecondary winding, the sinusoidal output voltage waves beingsubstantially evenly spaced over 360 degrees.
 2. The variable speeddrive system of claim 1, wherein the plurality of secondary windingscomprises a first, a second and a third secondary winding; and the firstsecondary winding comprises a phase angle shift of −θ with respect tothe second secondary winding, and a phase angle shift of +θ with respectto the third secondary winding.
 3. The variable speed drive system ofclaim 2, wherein the respective phase angle shift of the secondarywindings and their respective phase windings results in nine sinusoidaloutput voltage waves at output terminals of the phase windings.
 4. Thevariable speed drive system of claim 2, wherein the sinusoidal outputvoltage waves are spaced at about 40° angles of separation.
 5. Thevariable speed drive system of claim 1, wherein the converter isconfigured to rectify the nine sinusoidal output voltage waves togenerate an 18-pulse DC waveform supplied to the DC link.
 6. Thevariable speed drive system of claim 1, wherein the sinusoidal outputvoltage wave comprises 12-pulses and 6-phases to achieve a predeterminedripple factor at the DC link.
 7. The variable speed drive system ofclaim 1, wherein the sinusoidal output voltage wave comprises 24-pulsesand 12-phases to achieve a predetermined ripple factor at the DC link.8. The variable speed drive system of claim 1, wherein the primarywindings are wound for input terminals arranged for 575 Volts on a firstset of input taps, 460 Volts on a second set of input taps, and 400 to415 Volts on a third set of input taps, wherein any of the respectivevoltage levels may be applied at the respective input taps to provide afixed secondary voltage at the output terminals of the associatedsecondary windings.
 9. The variable speed drive system of claim 8,wherein the fixed secondary voltage is 580 VAC RMS voltage input to theconverter to provide 820 VDC on the DC link, and wherein the motor beinga four pole motor to be driven by the inverter over a range of at leasttwo times a power line input frequency.
 10. The variable speed drivesystem of claim 9, wherein the DC link voltage ranges from 813V to1000V, and the inverter output voltage is 575 volts.
 11. A variablespeed drive system configured to receive an input AC power at a fixed ACinput voltage magnitude and frequency and provide an output AC power ata variable voltage and variable frequency, the variable speed drivecomprising: a multiple pulse transformer, connected to a three-phase ACpower source providing the input AC voltage; a converter configured toconvert the input AC voltage to a DC voltage; a DC link connected to theconverter stage, the DC link being configured to filter and store the DCvoltage from the converter stage; and an inverter connected to the DClink, the inverter being configured to convert the DC voltage from theDC link into the output AC power having the variable voltage and thevariable frequency; the multiple pulse transformer comprising: threeprimary windings, each of the primary windings connected to a phase ofthe three-phase AC power source; each of the primary windings comprisingat least one input tap for connection to the AC power source; and aplurality of secondary phase windings, each secondary phase windinghaving multiple voltage output terminals.
 12. The variable speed drivesystem of claim 11, wherein each secondary phase winding has a phaseangle shift of θ with respect to a first adjacent secondary phasewinding, and has a phase angle shift of −θ with respect to a secondadjacent secondary winding.
 13. The variable speed drive system of claim12, wherein the phase angle shift between each of the secondary phasewindings generates nine sinusoidal voltage waves at the multiple voltageoutput terminals respectively of the secondary phase windings.
 14. Thevariable speed drive system of claim 13, wherein the respectivesinusoidal voltage waves are at substantially equally spaced phaseangles.
 15. The variable speed drive system of claim 13, wherein thephase angles are about 40° angles of separation between sinusoidalvoltage waves.
 16. The variable speed drive system of claim 13, whereinthe converter is configured to rectify the nine sinusoidal outputvoltage waves to generate an 18-pulse DC waveform supplied to the DClink.
 17. The variable speed drive system of claim 13, wherein theprimary windings are wound for a predetermined fixed input AC voltage,and the multiple output voltage terminals are arranged to output 575Volts on at least one of the multiple output voltage terminals, theoutput voltage terminals depending on the predetermined fixed input ACvoltage.
 18. A chiller system comprising: a refrigerant circuitcomprising compressor, a condenser, and an evaporator connected in aclosed refrigerant loop; and a multiple pulse transformer comprisingthree primary windings connected to a three-phase AC power source and aplurality of secondary windings connected to a variable speed drive topower a motor of the compressor; at least one of the primary windings orthe secondary windings comprising multiple taps for providing a multiplephase output voltage comprising a predetermined phase shift with respectto a corresponding winding of the remaining secondary windings; whereinthe phase shifting of the multiple phase output voltage provides threemultiple sinusoidal output voltage waves for each of the plurality ofsecondary windings with the multiple sinusoidal output voltage wavesbeing substantially evenly spaced over 360 degrees.
 19. The chillersystem of claim 18, wherein the primary winding comprising the multipletaps for receiving the three-phase AC power source, wherein the primarywindings are wound for input terminals arranged for 575 Volts on a firstset of input taps, 460 Volts on a second set of input taps, and 400 to415 Volts on a third set of input taps, wherein any of the designatedvoltage levels applied at the associated first, second or third inputtaps provide a fixed secondary voltage at the output terminals of theassociated secondary windings.
 20. The chiller system of claim 18,wherein the plurality of secondary winding each comprising three phasewindings, and each phase winding comprising the multiple taps foroutputting a multiple sinusoidal output voltage wave wherein themultiple sinsusoidal output voltage provides multiple sinusoidal outputvoltage waves for each of the plurality of secondary windings with themultiple sinusoidal output voltage waves substantially evenly spacedover 360 degrees.